24 



PHYSIOLOGICAL TRIGGERS 



Fig. 5. Electron photomicrographs ul .Xcrcis linibata sperm lixed in 5% formalin, diai>-ze(l 

 against distilled water, air dried to the collodion membrane and shadow cast with chromium. 



a and b, Control sperm from sea water suspension. In these the acrosome is a prominent 

 cone with a rather broad base, the nucleus is compact and oval and the midpiece is closely 

 applied to the nucleus. Popa's sensillae amoeboideae of the Nereis sperm midpiece are not 

 evident in these preparations. However, structures answering Popa's descrijjtion may be seen 

 on living Nereis sperm with phase oi)tics. c and d, Sperm with acrosome filaments from a 

 suspension treated with egg water. The filaments are of constant diameter and appear to have 

 undergone some disintegration at the tips. The nucleus in both c and d has rounded uj), the 

 midpiece has loosened and the base of the tail has looped about the head. All of these are 

 constant characteristics of the 'reacted' Nereis spermatozoan. 



